High-temperature fuel cells operate in an exothermic mode; for example, for solid-oxide fuel cells (SOFC) fueled with reformate, a mixture of primarily CO and H2, heat can be released during transport of oxygen across the solid electrolyte and from reaction of the transported oxygen with CO and H2 fuel. To avoid excessive temperatures that could damage the cell, active cooling is required. The extent of cooling is such that the amount of coolant required is a significant parasitic load on the cell that substantially reduces overall system efficiency.
Through direct fueling of a high-temperature fuel cell with CH4, heat adsorbing endothermic reactions can be introduced. As CO2 and H2O are formed in the cathode, these can react endothermically with CH4 to form additional CO and H2, providing additional fuel to the fuel cell while at the same time serving as a sink for thermal energy. Thus a desirable means of operating high temperature fuel cells is to provide CH4 as the fuel, instead of reformate, and allowing for internal reforming. In this way the parasitic cooling load requirements can be minimized.
However, there is no economic commercial source of methane gas; natural gas is an option. Most natural gases typically contain about 85-95% CH4, non-methane hydrocarbon components such as 1-5% C2H6, with lesser amounts of higher hydrocarbons, C3H8, C4H10, C5H12, and so on, plus in some cases N2, He, and CO2. At the conditions typical of high temperature fuel cells, these non-methane hydrocarbon components are well known to form carbonaceous deposits, tars, resins or solids with a very low H/C atom ratio, less than 1, in the anode, especially at the entrance sections of the anode where there can be local deficiencies in CO2 and H2O, as compared to the overall concentrations of these components in the anode portion of the fuel cell. Thus, while a desirable operation of high-temperature fuel cells is to directly fuel the anode with methane-rich natural gas, a means needs to be established to prevent carbonaceous deposits from forming. If natural gas or other hydrocarbon fuels can be converted to substantially pure methane, they can be used directly as fuel for high-temperature fuel cells.
U.S. Patent Publication No. 2005/0207970 Al, the disclosure of which is incorporated herein by reference, and related Patents EP 1586535A2 and EP 1616838 A2 disclose a process for pre-reforming of natural gas using a nickel-based catalyst. The process uses a H2O/C ratio of 3, and while claimed for natural gas, its utility is demonstrated only for pure methane. Since conversion of methane into nonhydrocarbons is reported, it can be concluded that the H/C ratio of hydrocarbons in the product, as compared to the feed, decreases.
U.S. Pat. No. 6,355,474, the disclosure of which is incorporated herein by reference, describes a process for pre-reforming of natural gas employing a noble metal based catalyst. However, the claimed process requires oxygen and an excessive amount of steam, and is therefore essentially a steam reforming process. While conversion of higher hydrocarbons is reported, a significant amount of methane conversion is also reported.
U.S. Pat. No. 4,844,837, the disclosure of which is incorporated herein by reference, describes a process for partial oxidation of hydrocarbons, essentially a pre-reformer for steam reforming. Catalyst compositions claimed include Pt, Pd, or Rh supported on alumina (Al2O3) stabilized with ceria (CeO2), lanthana (La2O3), baria (BaO), or chromia (Cr2O3). The claimed process requires a H2O/C ratio of at least 0.35 and an O2/C ratio of at least about 0.2. U.S. Pat. No. 3,642,460, and similar U.S. Pat. Nos. 4,417,905 and 3,882,636, the disclosures of which are incorporated herein by reference, describe natural gas or hydrocarbon pre-reforming processes, to produce a methane rich product gas. These are all essentially steam reforming processes, requiring excessive amounts of water. Likewise, U.S. Pat. No. 3,988,425, the disclosure of which is incorporated herein by reference, employs a small amount of H2 in addition to steam in the feed, which has a H2O/C ratio of 1.1-1.7.
Journal of Power Sources, 71 (1998), pp 315-320, describes reforming over conventional Raschig ring nickel based steam reforming catalysts, using a H2O/C ratio of 3. At 3.5 bars, conversions of 41% of CH4 and 73% of C2H6 were achieved at 1070/hr space velocity (measured at STP) and 690° C. Propane conversion was not measured
Journal of Power Sources, 86 (2000), pp 376-382, describes propane or butane fueled solid-oxide fuel cells in which the pre-reforming catalyst is ruthenium on Saffil non-woven alumina-silica (96/4) ceramic wool. The reported pre-reforming reaction is essentially partial oxidation, as the only oxygen-carrying species entering the pre-reformer is air.
Journal of Power Sources, 86 (2000), pp 432-441, describes steam reforming, combined steam and CO2 reforming, or simulated anode-tail gas reforming of natural gas using a nickel-based catalyst. In all cases, the feed H2O/C ratio is maintained at 2.5. For the case of anode tail gas reforming, methane conversions of 10-25% are reported, but there is no report of the conversion of the ethane or propane contents of the natural gas
Applied Catalysis A, 282 (2005), pp 195-204, reports pre-reforming of natural gas over a nickel-based catalyst, but the described process is steam reforming at a H2O/C ratio of from 0.74 to 3.4.